Xlpe/PVC Power Cable
Power cable is used for the transmission and distribution of electric power cables, power cables are often used in urban underground power grids, power station lines, industrial and mining enterprises internal power supply and underwater transmission lines across the river and sea.
In the power line, the proportion of cables is gradually increasing. Power cables are cable products used to transmit and distribute high-power energy in the backbone line of the power system, including 1-500KV and above various voltage levels, various insulated power cables.
HIstorical development
The use of power cables has been more than 100 years of history. In 1879, American inventor T.A. Edison wrapped jute around copper rods and threaded them into iron pipes, then filled them with asphalt mixture to make cables. He laid the cable in New York, pioneering underground power transmission. The following year, the British Callender invented asphalt impregnated paper insulated power cable. In 1889, the Englishman S.Z. Ferranti laid a 10 kV oil-impregnated paper insulated cable between London and Deptford. In 1908, Britain built a 20 kV cable network. Power cables are being used more and more widely. In 1911, Germany laid a 60 kV high voltage cable, which began the development of high voltage cables. In 1913, the German M. Hochstedt developed a component phase shielded cable, which improved the electric field distribution inside the cable and eliminated the tangential stress on the insulating surface, becoming a milestone in the development of power cables. In 1952, Sweden laid 380 kV ultra-high voltage cables in the northern power plant, which realized the application of ultra-high voltage cables. By the 1980s, it had been made into 1100 kV and 1200 kV UHV power cables.
Divided by insulation material
1. Oil-impregnated paper insulated power cable Power cable insulated with oil-impregnated paper. It has the longest application history. It is safe and reliable, long service life, low price. The main disadvantage is that the laying is limited by the drop. Since the development of non-dripping paper-impregnated insulation, the problem of drop limitation has been solved, so that oil-impregnated paper-insulated cables can continue to be widely used.
2. Plastic insulated power cable The insulation layer is extruded plastic power cable. Commonly used plastics are polyvinyl chloride, polyethylene, crosslinked polyethylene. The plastic cable has the advantages of simple structure, convenient manufacturing and processing, light weight, convenient laying and installation, and is not limited by laying drop. Therefore, it is widely used as a medium and low voltage cable, and has the trend of replacing viscous impregnated oil paper cables. Its biggest disadvantage is the presence of dendritic breakdown phenomenon, which limits its use at higher voltages.
3. The insulating layer of rubber insulated power cable is rubber plus various adjuvants, extruded on the conductive wire core after full mixing, and vulcanized by heating. It is soft and elastic, suitable for frequent movement and small bending radius of laying.
Commonly used as insulation materials are natural rubber - styrene butadiene rubber mixture, ethylene-polypropylene rubber, butyl rubber and so on.
Graded by voltage
1. Low-voltage cable: suitable for fixed laying in AC 50Hz, rated voltage 3kv and below on the transmission and distribution lines for electricity transmission.
2. Medium and low voltage cables: (generally 35KV and below) : PVC insulated cables, polyethylene insulated cables, cross-linked polyethylene insulated cables, etc.
3. High-voltage cable: (generally 110KV and above) : polyethylene cable and cross-linked polyethylene insulated cable.
4. Ultra-high voltage cable: (275 ~ 800KV).
5. Uhv cable: (1000KV and above).
Graded by voltage
According to the voltage level, it can be divided into medium and low voltage power cables (35 kV and below), high voltage cables (110 kV and above), ultra-high voltage cables (275 ~ 800 kV) and ultra-high voltage cables (1000 kV and above). In addition, it can also be divided into AC cables and DC cables according to the current system.
Divided by insulation material
1. Oil-impregnated paper insulated power cable Power cable insulated with oil-impregnated paper. It has the longest application history. It is safe and reliable, long service life, low price. The main disadvantage is that the laying is limited by the drop. Since the development of non-dripping paper-impregnated insulation, the problem of drop limitation has been solved, so that oil-impregnated paper-insulated cables can continue to be widely used.
2. Plastic insulated power cable The insulation layer is extruded plastic power cable. Commonly used plastics are polyvinyl chloride, polyethylene, crosslinked polyethylene. The plastic cable has the advantages of simple structure, convenient manufacturing and processing, light weight, convenient laying and installation, and is not limited by laying drop. Therefore, it is widely used as a medium and low voltage cable, and has the trend of replacing viscous impregnated oil paper cables. Its biggest disadvantage is the presence of dendritic breakdown phenomenon, which limits its use at higher voltages.
3. The insulating layer of rubber insulated power cable is rubber plus various adjuvants, extruded on the conductive wire core after full mixing, and vulcanized by heating. It is soft and elastic, suitable for frequent movement and small bending radius of laying.
Commonly used as insulation materials are natural rubber - styrene butadiene rubber mixture, ethylene-polypropylene rubber, butyl rubber and so on.
Graded by voltage
1. Low-voltage cable: suitable for fixed laying in AC 50Hz, rated voltage 3kv and below on the transmission and distribution lines for electricity transmission.
2. Medium and low voltage cables: (generally 35KV and below) : PVC insulated cables, polyethylene insulated cables, cross-linked polyethylene insulated cables, etc.
3. High-voltage cable: (generally 110KV and above) : polyethylene cable and cross-linked polyethylene insulated cable.
4. Ultra-high voltage cable: (275 ~ 800KV).
5. Uhv cable: (1000KV and above).sort
According to the voltage level, it can be divided into medium and low voltage power cables (35 kV and below), high voltage cables (110 kV and above), ultra-high voltage cables (275 ~ 800 kV) and ultra-high voltage cables (1000 kV and above). In addition, it can also be divided into AC cables and DC cables according to the current system.
Protective layer
The role of the protective layer is to protect the power cable from the intrusion of external impurities and moisture, and to prevent external forces from directly damaging the power cable. The protective layer is divided into an inner protective layer and an outer protective layer, which are used to protect the insulation layer. The quality of the protective layer is directly related to the service life of the cable. The concentric layer between the armor layer and the metal sheath is the protective layer. High voltage power cable padding is also a protective layer.
The basic structure of any kind of power cable is composed of three basic parts: conductive wire core, insulation layer and protective layer.
Conducting core
The wire core is the conductive part of the power cable, which is used to transmit electrical energy and is the main part of the power cable. The loss of the core is mainly determined by the cross section of the conductor and the conductivity coefficient of the material. In order to reduce the loss of the cable core, the cable core is generally made of copper or aluminum with high conductivity.
Insulating layer
The insulation layer is the electrical isolation between the wire core and the earth and the different phases of the wire core to ensure the transmission of electric energy, and is an indispensable part of the power cable structure. The insulation layer separates the core conductor from the protective layer to prevent leakage. The insulation layer is used to withstand the effect of voltage, its working field strength is very high, because the insulation layer will inevitably remain some bubbles, these bubbles under the action of a strong electric field, it is easy to be ionized and produce partial discharge, and accompanied by ozone corrosion of the insulation layer, so the insulation layer is required to have good corona resistance. At present, in the cable insulation layer materials of 110kV and below, crosslinked polyethylene dominates.
Shielding layer
15KV and above power cables generally have conductor shielding layer and insulation shielding layer.
Power cables of 6kV and above generally have conductor shielding layer and insulation shielding layer. The shielding layer shields the electromagnetic field generated by the current in the cable to protect peripheral components. When the cable is damaged, the leakage current will flow along the shielding layer to the grounding network, thus playing a protective function.
1. It takes up less space. Generally buried in the soil or laid indoors, trenches, tunnels, the insulation distance between the lines is small, do not use poles and towers, occupy less land, and basically do not occupy surface space.
2. High reliability. Little affected by climatic conditions and surrounding environment, stable transmission performance and high reliability.
3. It has more favorable conditions for the development of ultra-high pressure and large capacity, such as low temperature and superconducting power cables.
4. Large distributed capacitance.
5. Less maintenance work.
Technical characteristics
1.Can withstand high or even extremely high working voltage for a long time, should have very good electrical insulation performance.
2. Can transmit a large current (900 amps or even thousands of amPs), so it will use a conductive wire core with a cross-sectional area of hundreds or even thousands of square millimeters.
3. The power cable adopts a variety of combination of protective layer structure, which can adapt to various laying methods and use environment (underground, water trench pipe, tunnel, shaft).
In fact, the high-voltage insulation technology, large electric transmission technology, structural balance, and sheath structure that power cables rely on represent the academic level of these aspects of the electrical discipline.
1. Crosslinked polyethylene insulated cable has excellent dielectric properties, but poor resistance to corona and ionization discharge.
2. Polyethylene insulated cable has good process performance, easy processing, poor heat resistance, easy deformation, easy to delay burning, easy to crack.
3. Polyvinyl chloride insulated cable is chemically stable, non-combustible, and has sufficient material sources.
4.The higher insulation performance of power cables is oil-impregnated paper insulated cables.
1.Can withstand the power grid voltage. Including operating voltage, fault overvoltage and atmosphere, operating overvoltage.
2.Can transmit the power that needs to be transmitted. Includes current under normal and fault conditions.
3.It can meet the mechanical strength and curvability required for installation and use, and is durable and reliable;
4. Rich material sources, economy, simple process, low cost.
Current carrying capacity
1. The current carrying capacity of the power cable mainly depends on the maximum allowable temperature and the ambient temperature around the cable, the structural ruler of each part of the cable and the material characteristics and other factors. So that the stable temperature of the wire reaches the maximum allowable temperature of the cable, known as the allowable current or safe current.
2. The cross-linked polyethylene insulated cable conductor is allowed to work for a long time. The temperature is 90℃. The voltage class of the natural rubber insulated cable is 6kW, and the long-term allowable operating temperature of the cable conductor is 65℃. The voltage class of polyethylene insulated cable is 10k state, and the long-term allowable operating temperature of the cable conductor is 70 ° C. The voltage class of the polyethylene insulated cable is 6k, and the long-term allowable operating temperature of the cable conductor is 65℃.
Connom fault
Short-circuit fault: there are two-phase short circuit and three-phase short circuit, mostly caused by hidden dangers left in the manufacturing process.
Grounding fault: One or more cores of a cable break down to the ground. The insulation resistance is lower than 10 kω, which is called low-resistance grounding. The insulation resistance is higher than 10 kω, which is called high-resistance grounding. Mainly due to cable corrosion, lead cracks, dry insulation, joint technology and materials.
Linear fault: a cable core or several cores are completely broken or incomplete broken. Cables affected by mechanical damage, terrain changes, or short circuits can cause wire breakage.
Mixed faults: two or more of the above faults.
External damage: External damage may be suffered during the storage, transportation, laying and operation of the cable, especially the directly buried cable that has been run, which is vulnerable to damage in the ground construction of other projects. Such accidents tend to account for 50% of cable accidents. In order to avoid such accidents, in addition to strengthening the work quality of cable storage, transportation, laying and other links, it is more important to strictly implement the earth-moving system.
Protective layer corrosion: the electrochemical corrosion of the underground stray current or the chemical corrosion of non-neutral soil makes the protective layer ineffective and loses its protective effect on the insulation. The solution is to install drainage equipment in the stray current dense area; When the local soil on the cable line contains chemicals that damage the lead cover of the cable, the cable should be installed in the pipe, and the neutral soil should be used as the cable liner and cover, and the cable should be coated with asphalt.
Overvoltage, overload operation: improper selection of cable voltage, sudden high voltage intrusion during operation or long-term overload may damage the cable insulation strength and breakdown the cable. This needs to be solved in a timely manner by strengthening inspections and improving operating conditions.
Outdoor terminal head flooding: due to poor construction, insulation glue is not filled, resulting in terminal head flooding, and eventually explosion. Therefore, it is necessary to strictly implement the construction process rules and carefully check and accept; Strengthen inspection and timely maintenance. The oil leakage of the terminal head destroys the sealing structure, causes the loss and drying of the impregnating agent at the end of the cable, increases the thermal resistance, accelerates the aging of the insulation, easily absorbs moisture, and causes thermal breakdown. When oil leakage from the terminal head is found, inspection should be strengthened, and power failure should be re-done in serious cases.
Power plants, substations and industrial and mining enterprises are a large number of use of power cables, once the cable fire explosion, will cause serious fire and power failure, in addition, when the cable burning produce a lot of smoke and gas, not only pollute the environment, but also endanger human life safety. To this end, attention should be paid to the fire protection of power cables.
The insulation layer of the power cable is composed of various combustible substances such as paper, oil, hemp, rubber, plastic, and asphalt, so the cable has the possibility of fire and explosion. The causes of cable fire and explosion are:
(1) Short-circuit fault caused by insulation damage. The protective lead cover of the power cable is damaged during laying or the cable insulation is damaged by mechanical damage during operation, which causes the insulation breakdown between the cable and the lead cover, and the resulting arc causes the insulation material and the cable outer protective layer material to burn and catch fire.
(2) The cable is overloaded for a long time. Overload operation for a long time, the running temperature of the cable insulation material exceeds the maximum allowable temperature of normal heating, so that the insulation of the cable aging and drying, this phenomenon of insulation aging and drying, usually occurs in the entire cable line. Due to the aging and drying of the cable insulation, the insulation material loses or reduces the insulation and mechanical properties, so it is easy to break down and burn, and even burn at many places along the entire length of the cable at the same time.
(3) Oil immersed cable due to height difference leakage, oil leakage. When the height difference of oil immersed cable is large, the phenomenon of oil leakage may occur. As a result of the flow, the upper part of the cable dries up due to the loss of oil, and the thermal resistance of this part of the cable increases, causing the paper insulation to coking and breakdown in advance. In addition, because the upper oil flows downward, the upper cable head makes space and creates negative pressure, so that the cable is easy to absorb moisture and make the end of the moisture. Due to the accumulation of oil in the lower part of the cable, a large static pressure is generated, which causes the cable head to leak oil. Cable moisture and oil leakage increase the chance of fire failure.
(4) Insulation breakdown of the middle connector box. The middle joint of the cable joint box is oxidized, heated and gummy during operation due to weak crimping, weak welding or improper selection of joint materials; When making the cable intermediate joint, the quality of the insulating agent injected in the intermediate joint box does not meet the requirements. When the insulating agent is injected, there are pores in the box memory and the cable box is poorly sealed and damaged, and the moisture is leaked into the box. The above factors can cause the insulation breakdown, forming a short circuit, and causing the cable explosion and fire. Electrical automation technology network
(5) Cable head burning. Due to the moisture accumulation on the surface of the cable head, the porcelain bushing of the cable head is broken and the distance between the lead wires is too small, which leads to the flash fire, causing the surface insulation of the cable head and the lead wire insulation to burn.
(6) External fire and heat sources cause cable fires. Such as the spread of the fire of the oil system, the spread of the oil circuit breaker explosion fire, the spontaneous combustion of the boiler powder system or the coal transport system, the baking of the high-temperature steam pipeline, the chemical corrosion of the acid and alkali, welding sparks and other tinder, can make the cable fire.
Once the cable catches fire, the following methods should be used to extinguish it:
(1) Cut off the power supply of the fire cable. Cable fire burning, no matter what the cause, should immediately cut off the power, and then, according to the path and characteristics of the cable, carefully check to find out the fault point of the cable, and should quickly organize personnel to save.
(2) The fire in the cable trench is not a fault of the cable power supply. When the cables in the cable trench are on fire, if the cables laid side-by-side with the same trench have an obvious possibility of fire, the power supply of these cables should be cut off. If the cables are arranged in layers, first cut off the power supply of the heated cable above the fire cable, then cut off the power supply of the cable side by side with the fire cable, and finally cut off the power supply of the cable under the fire cable.
(3) Close the fire insulation door of the cable trench or block both ends of the cable trench. When the cable in the cable trench is on fire, in order to avoid air circulation and facilitate rapid fire extinguishing, the fire insulation door of the cable trench should be closed or the two ends should be blocked, and the method of suffocation should be used to extinguish the fire.
(4) Do a good job of personal protection when extinguishing cable fires. Because the cable fire burning will produce a lot of smoke and gas, extinguish the cable fire, the rescue personnel should wear a gas mask. In order to prevent human shock during the rescue process, rescue personnel should wear rubber gloves and insulating boots. If the high-voltage cable is found to be connected to the ground, rescue personnel should comply with the following rules: do not enter the indoor space within 4m away from the fault point, and do not enter the outdoor space within 8m away from the fault point, so as to avoid injury by stepping voltage and contact voltage. This does not apply to the rescue of injured persons, but protective measures should be taken.
(5) Fire extinguishing equipment used to extinguish cable fires. Fire extinguishers should be used to extinguish cable fires, such as dry powder fire extinguishers, "1211" fire extinguishers, carbon dioxide fire extinguishers, etc. It can also be covered with dry sand or loess; If you use water to extinguish the fire, it is best to use a spray gun; If the fire is fierce and cannot be put out by other means, after the power is cut off, water can be poured into the cable trench to seal the fault and extinguish the fire.
(6)When extinguishing a cable fire, it is forbidden to touch the cable steel armor and move the cable directly by hand.
